feat: initialize frontend unit test framework with Vitest
- Add vitest ^4.0.8, jsdom, @vitest/coverage-v8 to devDependencies
- Configure vitest.config.ts for jsdom environment with src/**/*.spec.ts pattern
- Simplify test-setup.ts (no TestBed needed for standalone unit tests)
- Add example spec file src/app/app.spec.ts (5 tests) covering App component logic
- Add npm scripts: test:unit, test:unit:watch, test:unit:coverage
- Verified: 5/5 frontend tests pass, 86/86 backend pytest tests still pass
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